Apparatus for a stent or other medical device having a bistable spring construction

ABSTRACT

A stent is described comprising a unit cell having a negative spring rate and a bistable function. The unit cell comprises first and second segments, the first segment being more rigid than the second segment. The second segment is coupled to the first segment, and the first segment comprises a substantially fixed sinusoidal shape. In the contracted state, the second segment is held stable in a sinusoidal shape, but when a force is applied the second segment bends out to a stable, convex-shaped deployed state. The second segments withstand the application of radial forces in the convex-shaped deployed state, and diametral recoil of the stent is minimized.

FIELD OF THE INVENTION

The present invention relates to stents, and more particularly, to aunit cell of a stent that is configured to snap between contracted anddeployed states using a first relatively rigid segment coupled to asecond relatively flexible segment.

BACKGROUND OF THE INVENTION

There are several kinds of stents on the market with either balloonexpandable or self-expanding function. Balloon expandable stents aregenerally made from a material that can easily be plastically deformedinto two directions. Before insertion, the stent is placed around theballoon section at the distal end of a catheter and pressed together toreduce the outer dimensions.

When the stent is delivered into the body in a desired location, it isexpanded and thereby plastically deformed to a larger diameter byinflating the balloon. Once expanded, the stent supports the surroundingtissue and prevents at least local narrowing of the vessel.

Such plastically deformable stents need to have sufficient rigidity inthe radial direction, but also some flexibility in the axial directionto enable delivery through tortuous anatomy. Furthermore, the amount ofmaterial should be as small as possible and the inner surface of thestent should not obstruct the flow through the channel (e.g., for blood)or cause too much turbulence.

Problems that generally occur after stent implantation are several:After crimping the stent onto the balloon of the delivery catheter, thestent experiences some elastic recoil to a slightly larger diameter,which can cause problems, e.g., snagging, when the catheter is advancedthrough the patient's vasculature. In addition, the engagement forcesbetween the balloon and stent can become so small that the stent slipsoff the catheter. Moreover, a large stent delivery profile reduces thenumber of situations in which the stent can be used.

Another problem with balloon expandable stents is recoil of these stentsafter deployment. In this case, after expansion by the balloon of thedelivery catheter, the stent outer diameter will shrink slightly oncethe balloon is deflated. The percentage change in deployed stentdiameter due to recoil can be as much as 10%, and can cause migration ofthe stent.

A self-expanding stent typically is made of a more or less elasticallyexpanding structure, which is affixed to the delivery catheter by someexternal means. For example, this type of stent is held in itsconstrained state by a delivery sheath that is removed at the moment ofstent deployment, so that the stent self-expands to its preferredexpanded form. Some of these stents are made of shape memory materialwith either superelastic behavior or temperature sensitive triggering ofthe expansion function.

A disadvantage of self-expanding stents is the need for the deliverysheath, thus resulting in a larger delivery profile. The removal of thesheath also requires a sheath retraction mechanism, which has to beactivated at the proximal end.

Most balloon expandable and self expanding stents further have thedisadvantage of that they experience large length changes duringexpansion and exhibit a poor hydrodynamic behavior because of the shapeof the metal wires or struts.

Still further balloon expandable stents exhibit a positive spring rate,which means that further diametral expansion can only be achieved byhigher balloon pressure. Moreover, previously-known stents typically areconstructed so that external forces, working on the stent in the radialdirection, may cause bending forces on the struts or wires of thestructure.

For example, a unit cell of a Palmaz-Schatz stent, as produced by theCordis division of Johnson & Johnson, or the ACT One Coronary stent,produced by Progressive Angioplasty Systems, Inc. have in theircontracted delivery state a flat, rectangular shape and in theirexpanded condition a more or less diamond-shaped form with almoststraight struts (Palmaz-Schatz) or more curved struts (ACT-One).

The shape of the unit cell of such stents is typically symmetrical withfour struts each having the same cross section. In addition, the loadingof the cell in the axial direction will typically cause an elastic orplastic deformation of all of the struts, resulting in an elongation ofthe unit cell in the axial direction. These unit cells have a positivespring rate. For stents based upon these unit cells, the stabilityagainst radial pressure is merely dependent on the bending strength ofthe struts and their connections.

In view of these drawbacks of previously known stents, it would bedesirable to provide a stent having minimal elastic spring back uponbeing compressed onto a balloon catheter.

It also would be desirable to provide a stent having minimal recoil sothat the stent remains at its selected deployed diameter afterexpansion.

It further would be desirable to provide a stent having a minimal lengthchange during deployment of the stent.

It still further would be desirable to provide a stent that is notcharacterized by a positive spring rate, so that achieving furtherexpansion does not require continually increasing balloon pressure.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide a stent having minimal elastic spring back upon being compressedonto a balloon catheter.

It is also an object of the present invention to provide a stent havingminimal recoil so that the stent remains at its selected deployeddiameter after expansion.

It is another object of the present invention to provide a stent havinga minimal length change during deployment of the stent.

It is another object of the present invention to provide a stent that isnot characterized by a positive spring rate, so that achieving furtherexpansion does not require continually increasing balloon pressure.

These and other objects of the present invention are achieved byproviding a stent comprising a unit cell having a negative spring rateand a bistable function. In the context of the present invention, thephrase “bistable function” means that the unit cell has only twoconfigurations in which it is stable without the need for an externalforce to hold it in that shape. In a preferred embodiment, the unit cellis formed using at least two different segments, wherein a first segmentis relatively rigid while a second segment is more flexible than thefirst segment.

The first segment preferably comprises a sinusoidal shape and does notsubstantially change in shape. The second segment is coupled to thefirst segment in such a way that the first segment inhibits deformationof the second segment in one direction. The second segment has only twostable positions, one in a contracted state and the other in a deployedstate.

In the contracted state, the second segment is held stable in asinusoidal shape when a compressive force is applied against the secondsegment in a direction toward the first segment. When a radially outwardforce is applied to the unit cell that is sufficient to displace thesinusoidal profile of the second segment, the second segment will buckleoutward from the first segment to a deployed state where it comprises aconvex shape. When the second segment is in any other position betweenthe contracted and deployed states it is unstable, and will return toeither the contracted or deployed state.

The stent as a whole therefore is deployed when the radially outwardforce, e.g., provided by a balloon, overcomes the resistance of one ormore second segments in one or more unit cells. The expansion of thesecond segments provides radial expansion of the stent, as the firstsegments of the unit cells do not substantially change in size or shape.

When a stent comprising the above-described unit cells is deployed to aselected deployed diameter, it reaches its maximum stability againstradial pressure. This makes the construction stronger than prior stentsbecause the second segments may withstand considerable radial forces intheir stable, convex-shaped deployed states.

Methods of actuating the apparatus of the present invention also areprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description of the preferred embodiments, in which:

FIGS. 1A-1B show the principle of a bistable mechanism;

FIG. 2 depicts the force-displacement characteristic of the mechanism ofFIG. 1B;

FIG. 3 depicts a bistable unit cell in accordance wits the presentinvention;

FIG. 4 depicts the force-displacement characteristic of the mechanism ofFIG. 3;

FIGS. 5A-5B shows two adjacent unit cells in accordance with the presentinvention in contracted and deployed states, respectively;

FIG. 6 shows a single circumferential ring of unit cells of a stent in astable, fully collapsed configuration;

FIG. 7 shows the circumferential ring of unit cells of FIG. 6 in astable, fully expanded configuration;

FIGS. 8A-8B depicts features of a plurality of unit cells in accordancewith the present invention in contracted and deployed states,respectively; and

FIGS. 9A-9B describe a preferred method of using a stent in accordancewith the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the operative principles of the stent of thepresent invention are described. In FIG. 1A, flexible rod 20 having alength L is affixed at each end by external clamps 22. When flexible rod20 is compressed along central axis A-A by a distance ΔL, it reaches itsbuckling stress and the central part of rod 20 then will bend out in asidewards direction, either to position 24 or 26, as shown in FIG. 1B.

Because the ends of rod 20 are held stable by external clamps 22, it ispossible to move the central section of rod 20 between two stablepositions 24 and 26. This movement is in a direction X that isperpendicular to central axis A-A of rod 20. All positions betweenstable positions 24 and 26 are unstable. In FIG. 1B, the central part ofrod 20 must be displaced at least a distance Δx before the rod can betransformed from stable position 24 to stable position 26.

FIG. 2 shows the force-displacement characteristics of rod 20 of FIG.1B. As rod 20 is moved from stable position 24, the force increasesrapidly from zero to Fmax. At that moment, rod 20 becomes unstable in aposition between stable positions 24 and 26, for example, in theposition depicted by the sinusoidal shape of rod 20 in FIG. 1B. At thispoint, further displacement in direction X requires less force becausethis spring system has a negative spring rate. The force even becomeszero in the mid position and further movement occurs automatically. Asseen in FIG. 2, the system of FIG. 1B is symmetrical and the forceneeded to move from lower position 26 back to upper position 24 has thesame characteristic.

Referring to FIG. 3, unit cell 30 constructed in accordance with thepresent invention comprises first segment 32 and second segment 34 thatis more flexible than first segment 32. First segment 32 functions as arelatively rigid clamp, like clamps 22 in FIG. 1B. First segment 32comprises a sinusoidal shape that does not substantially deform. Incontrast, second segment 34 acts as a flexible rod, like rod 20 of FIG.1B. Second segment 34 is coupled to first segment 32 by first and secondhinges 31, which may be either plastic or elastic, that are disposed atopposing ends of first segment 32.

Like rod 20 of FIG. LB, when the ends of second segment 34 are heldstable by hinges 31, it is possible to move the central section ofsecond segment 34 between two stable positions 36 and 38 (shown indotted line in FIG. 3). The movement occurs in a direction X that isperpendicular to central axis A-A, and all positions between stablepositions 36 and 38 are unstable. Second segment 34 is held stable inlower position 38 because it adapts to the sinusoidal profile of firstsegment 32 when compressed, and is clamped in that position by thecompressive forces exerted by coupling each end of second segment 32 tohinges 31.

With respect to FIG. 4, second segment 34 displays an asymmetricforce-displacement characteristic. To initially displace second segment34 from stable upper position 36 requires a compressive starting forceF_(C). To displace second segment 34 from stable lower position 38requires deployment force F_(D), which may be less than force F_(C).Deployment force F_(D) may be made as small as desired, even zero ornegative, but needs to have a positive value if lower position 38 is tobe stable.

The application of forces F_(C) and F_(D) serve to transform secondsegment 34 between stable contracted and deployed states. The forcerequired to transform second segment 34 between its two stable statesdefines the force-displacement characteristic of unit cell 30. As willbe described hereinbelow, a stent having a plurality of unit cells 30may have different force-displacement characteristics for eachindividual unit cell, to selectively deploy certain unit cells whileothers remain contracted.

First segment 32 of FIG. 3 preferably has a larger cross-section thansecond segment 34 so that it is more rigid. Alternatively, instead ofusing segments of different cross-section, the two segments in each unitcell 30 may have the same cross-sections but exhibit different strengthsor rigidity and still accomplish the same effect. One way to obtain suchdifferences in strength or rigidity would be to use different materialsfor the segments. Another way would be to use the same material, such asa metal, for all the segments but selectively strengthen (e.g., by heattreating) first segment 32.

It should be noted that heat treatment will not strengthen allmaterials. Nitinol, for example, becomes more pliable as a result ofheat treatment. This property of Nitinol can be exploited, however, torender one section of Nitinol more pliable relative to a second,non-heat-treated section of Nitinol.

There are several ways to manufacture unit cell 30 of a stent of thepresent invention. The device may be manufactured from an arrangement ofwire or strip, welded together at specific places, e.g., hinges 31.Alternatively, metal may be deposited in the desired pattern onto asubstrate or prealloy powder may be sintered. Alternatively, the devicemay comprises a tubular material, and a pattern of slits or slots may bemade in the wall by means of etching, grinding, cutting (e.g., with alaser, water, etc.), spark erosion or any other suitable method. Thepattern also may be made formed as a flat plate and teen welded, brazedor crimped to a more or less cylindrical shape or a cylindrical midsection with two conical ends of enlarged diameter.

Materials that may be used to construct a stent comprising unit cell 30include polymers, composites, conventional metals and shape memoryalloys with superelastic behavior or with temperature sensitivebehavior, or a combination of two or more of these materials.

With respect to FIG. 5, a preferred arrangement of two adjacent unitcells in accordance with the present invention is described, whereinhorizontal line A-A is parallel to the central axis of a stent. A firstunit cell comprises first segment 50 and second segment 40, while thesecond adjacent unit cell comprises second segment 42 and first segment52. Second segments 40 and 42 are more flexible than first segments 50and 52, respectively, and second segments 40 and 42 are coupled to theirrespective first segments at hinges 46.

These adjacent unit cells preferably are arranged so that secondsegments 40 and 42 are disposed between first segments 50 and 52, asshown in FIG. 5A. Second segments 40 and 42 preferably are connected byjoint 44 that is disposed near a midpoint of second segments 40 and 42.In FIG. 5A, the sinusoidal configurations of rigid first segments 50 and52 serve to hold flexible second segments 40 and 42 in stable,sinusoidally-shaped contracted states.

Referring to FIG. 5B, the adjacent unit cells of FIG. 5A are depicted ina stable deployed state. The unit cells preferably are deployed byapplying uniform radially outward force F_(D), e.g., by inflating aballoon, that is sufficient to overcome the resistance of secondsegments 40 and 42 in their stable, sinusoidal-shaped contracted states.Once force F_(D) has overcome this resistance, second segments 40 and 42will automatically snap into their respective stable, convex-shapeddeployed positions, as shown in FIG. 5B.

FIG. 6 shows the general appearance of a circumferential ring of atubular stent constructed in accordance with the present invention inits fully contracted configuration. Ring 60 comprises a plurality ofunit cells, each unit cell comprising second segment 62 that is moreflexible than first segment 64. First and second segments 64 and 62 arecoupled together by flexible hinges 61, while adjacent second segmentsare connected by joints 63. In FIG. 7, circumferential ring 70 of anillustrative stent is shown in a fully deployed state. Second segments72 of ring 70 have been deployed and assume stable, convex shapes.Second segments 72 provide the radial expansion of ring 70, while firstsegments 74 substantially maintain their original shapes. Hinges 71 ofFIG. 7 couple first and second segments 74 and 72, while joints 73connect adjacent second segments 72.

Referring to FIG. 8, stent 80 constructed of a series of threecircumferential rings 60 is depicted, for illustrative purposes,flattened. In three-dimensions, stent 80 would extend circumferentiallyabort central axis A-A to form an extended tubular shape similar tocomprising a series of circumferential rings as depicted in FIGS. 6-7,such that segments 100 and 103 of stent 80 are in effect the samesegment.

In FIG. 8A, stent 80 is illustrated in a contracted state. Stent 80comprises first segments 100, 101, 102 and 103, and further comprisessecond segments 81, 82, 83, 84, 85 and 86 that are more flexible thanfirst segments 100-103. First segments 100-103 substantially maintaintheir original shape. There preferably are two second segments disposedbetween every two first segments, as depicted in FIG. 8A. Joints 92connect adjacent second segments, while hinges 93 connect first andsecond segments. Joints 92 and hinges 93 are disposed at approximatelythe same distance apart as they longitudinally alternate along axis A-A.

Stent 80 is contracted by applying a compressive force F_(C), e.g., theforce applied by the fingers of a physician, as shown in FIG. 8A.Compressive force F_(C) contracts second segments 81 and 82 into stable,sinusoidal shapes between first segments 100 and 101. Compressive forceF_(C) also contracts second segments 83 and 84 into stable, sinusoidalshapes between first segments 101 and 102, and further contracts secondsegments 85 and 86 into stable, sinusoidal shapes between first segments102 and 103.

The resistive force that second segments 81-86 provide in their stable,sinusoidal-shaped contracted state may be overcome by radially outwardforce F_(D), which is perpendicular to axis A-A, as shown in FIG. 8B.Second segments 81-86 snap from their contracted states to stable,convex-shaped deployed states when force F_(D) is applied, as shown inFIG. 8B.

Referring now to FIG. 9, arm exemplary method of using stent 80 of FIGS.8A-8B is described. In FIGS. 9A-9B, it should be noted that stent 80 isillustratively depicted from a side view as having a preferred geometryand thickness, whereas the same stent in FIGS. 8A-8B was depicted asflattened for illustrative purposes.

In a first method step shown in FIG. 9A, stent 80 is compressed ontoballoon 122 of conventional balloon catheter 120, e.g., by applying acompressive force manually. Catheter 120 is inserted into a patient'svessel, preferably over guidewire 124, and a distal region of catheter120 having balloon 122 is positioned within treatment vessel V. Thedistal region of catheter 120 preferably is positioned under fluoroscopyusing at least one radiopaque marker band (not shown) disposed on thedistal region of catheter 120.

When catheter 120 is properly positioned, e.g., within stenosed regionS, balloon 122 is inflated to cause one or more second segments 81-86 todeploy to a convex shape bowed away from first segments 100-103, asshown in FIG. 9B. Specifically, balloon 122 provides a radially outwardforce, described hereinabove with respect to FIG. 8B, that overcomes theresistive force provided by one or more second segments 81-86 in thecontracted state. Having flexible second segments 81-86 snap intoexpanded stable positions provides a stent with an extremely rigidsurface at all diameters that is able to better withstand externalforces than previously known stents.

The flexibility of stent 80 may be increased by disconnecting severalunit cells from their neighbor unit cells, for example, by cutting thecenter of one or more hinges 93. Another way to increase flexibility isto change the geometry of various segments within selected unit cellsalong axis A-A. In other words, referring to FIG. 8B, one or more secondsegments 81-86 could be constructed with larger and smaller diameter (orotherwise flexible and rigid) segments that alternate after each hinge93. In addition, varying the properties of second segments 81-86 in oneor more selected unit cells, e.g., increasing or decreasing thedeployment force for specific unit cells, stent 80 may be made capableof attaining different diameters in the deployed state, depending on theamount and location of unit cells that are transformed to the deployedstate.

Stent 80 may achieve a range of diameters by deploying selected unitcells in a stepwise fashion. In one scenario, the diameter of stent 80may be increased incrementally by varying the properties of secondsegments 81-86 to cause some rows of the stent to expand preferentiallybefore other rows. For example, as balloon 122 is inflated at arelatively low balloon pressure, only unit cells in the row of secondsegment 81 will deploy. Then, as balloon 122 further is inflated, onlythose unit cells in the row of second segment 82 nay deploy for asomewhat higher balloon pressure, and so forth, until the desired numberof rows have been deployed to achieve the desired stent diameter. Inthis manner, stent 80 may be suitable for use in a wide range ofvessels.

Furthermore, stent 80 may comprise different external diameters alongits length to conform to particular cavities. This is achieved byvarying the properties of second segments 81-86 along central axis A-Aof stent 80. For example, hinges 93 may be used to divide stent 80 intoa plurality of distinct sections, e.g., first end 110, second end 114and intermediate section 112. The unit cells within first end 110comprise second segments 81 and 82 that exhibit a firstforce-displacement characteristic. The unit cells within second end 114may comprise second segments 85 and 86 that exhibit secondforce-displacement characteristics, while the unit cells withinintermediate section 112 comprise second segments 83 and 84 having yetdifferent force-displacement characteristics.

The force-displacement characteristics of each unit cell may betailored, for example, such that second segments 81 and 82 may easilydeploy with little balloon pressure, while second segments 83-86 do notdeploy for such balloon pressure. This provides stent 80 having adeployed first end 110 and contracted intermediate section 112 andsecond end 114. To provide a progressively smaller, stent, secondsegment 83 may be configured to deploy within intermediate section 112while second segment 84 is not configured to deploy when the same forceis applied. This provides partial deployment within intermediate section112 and provides an intermediate diameter. Alternatively, all unit cellswithin first end 110 and second end 114 may be deployed while unit cellswithin intermediate section 112 remain partially or fully contracted toprovide a generally hourglass-shaped stent along axis A-A.

The above examples describe a few variations in stent configurations byvarying the force-displacement characteristics of individual unit cells.The present invention is intended to cover the numerous other stentconfigurations that can be attained when the unit cells selectivelydeploy as particular forces are applied.

Additionally, the overall stent diameter in the deployed state furthermay be varied by providing first and second segments having differentlengths, because relatively long second segments may bow away from theirrespective first segments a greater distance than smaller secondsegments. Also, stent characteristics may be varied when certainsections of the stent comprise a different number of unit cells relativeto other sections.

1. A stent comprising a plurality of unit cells, each unit cellcomprising a first segment having proximal and distal ends and asubstantially sinusoidal shape, and a second segment having proximal anddistal ends, the proximal end of the first segment coupled to theproximal end of the second segment, the distal end of the first segmentcoupled to the distal end of the second segment, the second segmentbeing more flexible than the first segment, wherein the unit cell has astable contracted state in which the second segment substantiallyconforms to the sinusoidal shape of the first segment, and a deployedstate in which the second segment has a convex shape bowed away from thefirst segment.
 2. The stent of claim 1 wherein the second segment ofeach unit cell is coupled to the first segment so that the first segmentinhibits deformation of the second segment in the contracted state. 3.The stent of claim 1 or 2 wherein the second segment of each unit cellis stable only in the contracted and deployed states.
 4. The stent ofclaim 1, 2 or 3 wherein the first segment of each unit cell issubstantially rigid.
 5. The stent of claim 1, 2, 3 or 4 wherein thefirst segment of each unit cell comprises a larger cross-sectional areathan the second segment.
 6. The stent of claim 1, 2, 3, 4 or 5 whereinthe first and second segments of each unit cell are manufactured usingdifferent materials.
 7. The stent of any one of claims 1 to 6 whereinthe proximal and distal ends of the first and second segments of eachunit cell are coupled together by hinges.
 8. The stent of claim 7wherein the hinges of each unit cell are elastic hinges.
 9. The stent ofclaim 7 wherein the hinges of each unit cell are plastic hinges.
 10. Thestent of any one of claims 1 to 9 wherein the unit cells are transformedfrom the contracted state to the deployed state by application of auniform radially outwardly directed force to an interior surface of thestent.
 11. The stent of any one of claims 1 to 10 where a first subsetof the plurality of unit cells has a second segment with a firstcross-sectional area and a second subset of the plurality of unit cellshas a second segment with a second cross-sectional area.
 12. The stentof any one of claims 1 to 11 wherein the plurality of unit cells arearranged in a longitudinally arranged series of circumferential rings.13. The stent of any one of claims 1 to 12 wherein the stent is capableof attaining different outer diameters depending on the amount andlocation of unit cells that are transformed to the deployed state. 14.The stent of any one of claims 1 to 13 wherein the unit cells aredesigned and arranged to provide a range of diameters for the stent in astepwise fashion.
 15. The stent of claim 14 wherein the stent has aninitial diameter at a first end, a final diameter at a second end, andat least one intermediate diameter between the first and second ends,the intermediate diameter differing from the initial and finaldiameters.
 16. The stent of claim 15 wherein the initial and finaldiameters are the same.
 17. The stent of any one of claims 12 to 16wherein within a circumferential ring, a first subset of unit cells hasa different force-displacement characteristic than a second subset ofthe plurality of unit cells.
 18. The stent of any one of claims 1 to 17wherein the second segment of each unit cell is coupled to the secondsegment of an adjacent cell.
 19. The stent of claim 18 wherein thesecond segment of each unit cell is coupled to the second segment of anadjacent cell by a joint disposed near a midpoint of the secondsegments.
 20. The stent of any one of claims 1 to 19 wherein the stentis made from a polymer, a metal, a composite, a shape memory materialwith superelastic behavior, a shape memory material with temperaturesensitive behavior, or a coordination of two or more of these materials.21. A method for deploying a stent having two substantially stablestates, the method comprising: providing a stent comprising a pluralityof unit cells in a contracted state, wherein each unit cell comprises afirst segment having proximal and distal ends and a substantiallysinusoidal shape, and a second segment having a proximal end that iscoupled to the proximal end of the first segment and a distal end thatis coupled to the distal end of the first segment, the second segmentbeing more flexible than the first segment, wherein the second segmentsubstantially conforms to the sinusoidal shape of the first segment inthe contracted state; and deploying at least one of the unit cells ofthe stent by causing the second segment of the unit cell to deploy to aconvex shape bowed away from the first segment of the unit cell.
 22. Themethod of claim 21 wherein the stent is provided in the contracted stateby compressing the stent onto a balloon of a balloon catheter.
 23. Themethod of claim 22 wherein at least one unit cell is deployed byapplying a radially outward force by inflating the balloon.
 24. Themethod of claim 21, 22 or 23 wherein unit cells of the stent aredeployed in a stepwise fashion.
 25. The method of claim 21, 22, 23 or 24wherein the number of unit cells that are deployed is proportionate to aradially outward force that is applied to the stent.
 26. The method ofany one of claims 21 to 25 wherein unit cells are selectively deployedby providing second segments having varying diameters.
 27. The method ofany one of claims 21 to 26 wherein the diameter of the stent in adeployed state is varied by varying lengths of first and second segmentsof a unit cell.
 28. The method of any one of claims 21 to 27 wherein thediameter of the stent in a deployed state is varied by varying thenumber of unit cells that are provided in the contracted state.